The Coulter principle, also known as electronic sensing zone technology, is a well established method of characterizing the size and number of particles in a liquid sample. According to the Coulter principle, particles can be characterized by their effect on a current-induced movement of electrolyte through a small aperture as the particle passes through the aperture. Entry of a particle into the aperture displaces some of the charged electrolyte in the aperture, causing an increased electrical resistance across the aperture, resulting in an increased voltage measured across the aperture when current is held constant. As the particle exits the aperture, normal flow of electrolyte through the aperture resumes, resulting in a return to the starting voltage across the aperture before the particle entered. In this way, passage of a single particle through the aperture is identified by a characteristic voltage pulse across the aperture. The height of this voltage pulse is generally dependent on the size of the particle, since a larger particle will displace more electrolyte in the aperture, resulting in a larger voltage difference when the particle enters and passes through the aperture.
A Coulter Counter® is a particle characterization device that uses the Coulter principle to determine the number and sizes of particles in a liquid sample. The counter device comprises two liquid-containing chambers separated by a wall, with an aperture in the wall that allows electrolyte and particles in the liquid of the chambers to move from one chamber to the other. A pair of electrodes connected to a power source and disposed across the aperture, one in each chamber, provides a voltage difference across the aperture. Electrolytes in the liquid move from one chamber to the other in response to the applied voltage, generating an electric current. An applied force, such as a vacuum or the movement of a piston, causes the liquid to move from one chamber to the other. A detector monitors the voltage across the aperture, and a processor analyzes the voltage changes as liquid, electrolyte, and particles pass through the aperture from one chamber to the other, identifying and characterizing individual particles based on the characteristic voltage variation as the particles pass through the aperture.
Coulter Counters® are commonly used to characterize the number and types of cells in a biological sample, determining, for example, the number of red blood cells, white blood cells, and platelets in a blood sample. A Coulter Counter® can also be used in non-biological applications, characterizing the number and size-distribution of particles dispersed in any suitable sample.
The size of the aperture in a Coulter Counter® is selected to be larger than the particles suspected of being present in the liquid, yet small enough so that generally particles pass through the aperture one at a time. During a particle-characterization run, it is possible for the aperture to become blocked, typically by sample particles in the liquid that are larger than the aperture, by foreign particles such as dust or debris in the liquid, by aggregates of sample particles in the liquid, or by multiple particles passing through the aperture at the same time. If the aperture becomes blocked during a run, the run must typically be stopped and the blockage removed before particle analysis can continue. There are currently several methods of identifying blockage of the aperture during a run. A magnified window focused on the aperture can allow visual identification of a blockage. Alternatively, a magnified image of the aperture, such as one created by a CCD device focused on the aperture, can be monitored during the run. Both these methods depend on the presence of a user to continuously monitor a run, with the disadvantage that it may be difficult to maintain an optical focus on the aperture, and blockage of the aperture may be difficult to see, whether through a window or in a CCD image.
Automated methods of identifying blockage in an aperture of a Coulter Counter® type particle characterization device have been disclosed. U.S. Pat. No. 4,412,175; U.S. Pat. No. 4,450,435; U.S. Pat. No. 4,775,833; U.S. Pat. No. 6,389,912; and WO 2007/033669 disclose methods of detecting aperture blockage based on the width of the voltage pulse associated with passage of a particle through the aperture. The voltage across the aperture increases as a particle enters, remains high as the particle passes through, then returns to the initial level as the particle exits the aperture. A particle that blocks the aperture, or one that becomes entrapped in the aperture, is prevented or delayed from exiting the aperture, causing a sustained higher voltage across the aperture characterized by an increased pulse-width. It may be difficult, however, for this method to detect a partial or transient blockage of the aperture, or to distinguish blockage from other factors affecting aperture voltage, such as the presence of air in the sample.
Therefore, a need exists for a more reliable, automated method of detecting blockage in the aperture of a Coulter Counter® type particle characterization device. The present disclosure addresses that need by providing a particle characterization device adapted to more reliably and automatically detect both partial and complete blockages of the aperture.